This invention relates to recovering a pumpable crude oil from a reservoir of heavy oil or bitumen by the steam-assisted gravity drainage (SAGD) process, and more particularly to solvent deasphalting to remove an asphaltene fraction from the heavy oil or bitumen to yield the pumpable synthetic crude, and to combusting the asphaltene fraction to supply heat for generation of the injection steam.
Heavy oil reservoirs contain crude petroleum having an API gravity less than about 10 which is unable to flow from the reservoir by normal natural drive primary recovery methods. These reservoirs are difficult to produce due to very high petroleum viscosity and little or no gas drive. Bitumen, usually as tar sands, occur in many places around the world.
The steam-assisted gravity drainage (SAGD) process is commonly used to produce heavy oil and bitumen reservoirs. This generally involves injection of steam into an upper horizontal well through the reservoir to generate a steam chest that heats the petroleum to reduce the viscosity and make it flowable. Production of the heavy oil or bitumen is from a lower horizontal well through the reservoir disposed below the upper horizontal well.
Representative references directed to the production of crude petroleum from tar sands include Canadian Patent Application 2,069,515 by Kovalsky; U.S. Pat. No. 5,046,559 to Glandt; U.S. Pat. No. 5,318,124 to Ong et al; U.S. Pat. No. 5,215,146 to Sanchez; and Good, xe2x80x9cShell/Aostra Peace River Horizontal Well Demonstration Project,xe2x80x9d 6th UNITAR Conference on Heavy Crude and Tar Sands (1995), all of which are hereby incorporated herein by reference. Most of this technology has been directed to improving reservoir production characteristics. Surprisingly, very little attention has been directed to incorporating on-site downstream processing into the upstream field processing of the heavy oil or bitumen for improving the efficiency of operation and overall field production economy.
The heavy oil or bitumen produced by the SAGD and similar methods requires large amounts of steam generated at the surface, typically at a steam-to-oil ratio (SOR) of 2:1, i.e. 2 volumes of water have to be converted to injection steam for each volume of petroleum that is produced. Usually natural gas is used as the fuel source for firing the steam boilers. It is very expensive to supply the natural gas to the boilers located near the injection wells, not to mention the cost of the natural gas itself.
Another problem is that when the heavy oil or bitumen is produced at the surface, it has a very high viscosity that makes it difficult to transport and store. It must be kept at an elevated temperature to remain flowable, and/or is sometimes mixed with a lighter hydrocarbon diluent for pipeline transportation. The diluent is expensive and additional cost is incurred to transport it to the geographically remote location of the production. Furthermore, aspahaltenes frequently deposit in the pipelines through which the diluent/petroleum mixture is transported.
There is an unmet need in the art for a way to reduce the cost of steam generation and the cost and problems associated with heavy oil and/or bitumen surface processing and transporting. The present invention is directed to these unfulfilled needs in the art of SAGD and similar heavy oil and/or bitumen production.
The present invention provides a process and systems for producing heavy oil or bitumen economically by steam-assisted gravity drainage (SAGD), upgrading the heavy oil or bitumen into a synthetic crude, and using the bottom of the barrel to produce steam for injection into the reservoir.
Broadly, the present invention provides a process for recovering a pumpable synthetic crude oil from a subterranean reservoir of heavy oil or bitumen, comprising the steps of: (a) injecting steam through at least one injection well completed in communication with the reservoir to mobilize the heavy oil or bitumen; (b) producing the mobilized heavy oil or bitumen from at least one production well completed in the reservoir; (c) fractionating the heavy oil or bitumen produced from step (b) at a location adjacent to the reservoir into a first fraction as a minor amount of the heavy crude comprising a gas oil fraction and second fraction comprising a residue; (d) solvent deasphalting the second fraction from step (c) to form an asphaltene fraction and a deasphalted oil fraction essentially free of asphaltenes; (e) combusting the asphaltene fraction from step (d) to produce the steam for injection step (a); and (e) blending the first fraction from step (c) with the deasphalted oil fraction from step (d) to form a pumpable synthetic crude oil. The fractionation is preferably performed under atmospheric pressure. The asphaltene fraction from step (d) can be supplied as a liquid to the combustion step (e), or alternatively the asphaltene fraction from step (d) can be pelletized to obtain asphaltene pellets for supply to the combustion step (e).
The combustion step (e) preferably comprises combustion of the asphaltenes in a boiler to produce the injection steam for step (a). By this process, the solvent deasphalting step (d) can be performed at a first location to which the produced heavy oil or bitumen is transported, and the asphaltene fraction can be transported from the first location to a plurality of boilers spaced away from the first location, preferably adjacent to the injection well or wells. The boiler is preferably a circulating fluid bed boiler.
In an alternate embodiment, the combustion step (e) comprises gasification of the asphaltene fraction to produce a synthesis gas and the injection steam for step (a). The process can include recovering CO2 from the synthesis gas and injecting the CO2 into the reservoir. A portion of the steam produced from gasification can be expanded in a turbine to generate electricity.
Another aspect of the invention is a process for recovering a pumpable crude oil from a subterranean reservoir of heavy oil or bitumen. The process comprises the steps of: (a) injecting steam through one or more injection wells completed in communication with the reservoir to mobilize the heavy oil or bitumen; (b) producing the mobilized heavy oil or bitumen from at least one production well completed in the reservoir; (c) solvent deasphalting at least a portion of the heavy oil or bitumen produced from step (b) to form an asphaltene fraction and a deasphalted oil fraction essentially free of asphaltenes; (d) pelletizing the asphaltene fraction from step (c) to obtain asphaltene pellets; and (e) combusting the asphaltene pellets from step (d) to produce the steam for injection step (a). The combustion step (e) in one embodiment comprises combustion in at least one boiler to produce the injection steam for step (a). In one embodiment, the solvent deasphalting step (d) is preferably performed at a first location and the asphaltene fraction is transported from the first location to a plurality of boilers spaced away from the first location adjacent to the one or more injection wells. The at least one boiler is preferably a circulating fluid bed boiler. In an alternate embodiment, the combustion step (e) comprises gasification of the asphaltene pellets to produce a synthesis gas and the injection steam for step (a). The process can include the steps of recovering CO2 from the synthesis gas and injecting the CO2 into the reservoir with the steam. A portion of the steam generated from gasification can be expanded in a turbine to generate electricity.
Another aspect of the invention is the provision of a system for producing a pumpable synthetic crude oil. The system includes a subterranean reservoir of heavy oil or bitumen; at least one injection well completed in the reservoir for injecting steam into the reservoir to mobilize the heavy oil or bitumen; at least one production well completed in the reservoir for producing the mobilized heavy oil or bitumen; an atmospheric flash unit for fractionating the heavy oil or bitumen produced from the at least one production well into a minor portion comprising a gas oil fraction and a major portion comprising a residue fraction; a solvent deasphalting unit for separating the residue fraction into a minor portion comprising an asphaltene fraction and a major portion comprising a deasphalted oil fraction essentially free of asphaltenes; mixing apparatus for mixing the gas oil fraction and the deasphalted oil fraction to form a pumpable synthetic crude; a pelletizer for palletizing the asphaltene fraction into solid pellets; at least one boiler for combustion of the asphaltene pellets to generate the injection steam; and at least one line for supplying the steam from the at least one boiler to the at least one injection well.
A further aspect of the invention is the provision of a process for recovering a pumpable crude oil from a subterranean reservoir of heavy oil or bitumen. The process comprises the steps of: (a) injecting steam through one or more injection wells completed in communication with the reservoir to mobilize the heavy oil or bitumen; (b) producing the mobilized heavy oil or bitumen from at least one production well completed in the reservoir; (c) solvent deasphalting a first portion of the heavy oil or bitumen at a location adjacent to the reservoir to form an asphaltene fraction and a deasphalted oil fraction essentially free of asphaltenes; (d) combusting the asphaltene fraction from step (c) to produce the steam for injection step (a); (e) blending a second portion of the heavy oil or bitumen with the deasphalted oil fraction from step (c) to form a pumpable synthetic crude oil; and (g) pipelining the synthetic crude oil to a location remote from the reservoir.
In another aspect, the present invention provides a system for producing a pumpable synthetic crude oil. The system includes a subterranean reservoir of heavy oil or bitumen, at least one injection well completed in the reservoir for injecting steam into the reservoir to mobilize the heavy oil or bitumen, and at least one production well completed in the reservoir for producing the mobilized heavy oil or bitumen. An atmospheric flash unit is used to fractionate the heavy oil or bitumen produced from the production well into a minor portion comprising a light gas oil fraction and a major portion comprising a residue fraction. A solvent deasphalting unit separates the residue fraction into a minor portion comprising an asphaltene fraction and a major portion comprising a deasphalted oil fraction essentially free of asphaltenes. A mixing apparatus is provided for mixing the light gas oil fraction and the deasphalted oil fraction to form a pumpable synthetic crude. A boiler burns the asphaltene fraction as fuel to generate the injection steam. A line supplies the steam from the boiler to the injection well or wells.
The system can include a line for supplying the asphaltene fraction in liquid form to the boiler. Alternatively, a pelletizer unit can be used to form the asphaltene into solid pellets. The pelletizer unit preferably comprises: (1) an upright pelletizing vessel having an upper prilling zone, a sphere-forming zone below the prilling zone, a cooling zone below the sphere-forming zone, and a lower aqueous cooling bath below the cooling zone; (2) a centrally disposed prilling head in the prilling zone rotatable along a vertical axis and having a plurality of discharge orifices for throwing asphaltene radially outwardly, wherein a throw-away diameter of the prilling head is less than an inside diameter of the pelletizing vessel; (3) a line for supplying the asphaltene fraction in liquid form to the prilling head; (4) a vertical height of the sphere-forming zone sufficient to allow asphaltene discharged from the prilling head to form substantially spherical liquid pellets; (5) nozzles for spraying water inwardly into the cooling zone to cool and at least partially solidify the liquid pellets to be collected in the bath; (6) a line for supplying water to the nozzles and the bath to maintain a depth of the bath in the pelletizing vessel; (7) a line for withdrawing a slurry of the pellets in the bath water; and (8) a liquid-solid separator for dewatering the pellets from the slurry.
The atmospheric fractionator unit, the solvent deasphalting unit and the pelletizer are preferably centrally located with a plurality of the boilers located away from the central location adjacent to injection wells.
In an alternate embodiment of the heavy oil or bitumen production system, a slurrying unit is used for pelletizing the asphaltene fraction and forming an aqueous slurry which is supplied to a gasification unit for partial oxidation of the slurry to form a synthesis gas and generating the steam. A line supplies the steam from the gasification unit to the injection well or wells. The slurrying unit can include: (1) an upright prilling vessel having an upper prilling zone, a hot discharge zone below the prilling zone, a cooling zone below the discharge zone, and a lower cooling bath below the cooling zone; (2) a centrally disposed prilling head in the prilling zone rotatable along a vertical axis and having a plurality of discharge orifices for throwing asphaltene radially outwardly, wherein a throw-away diameter of the prilling head is less than an inside diameter of the prilling vessel; (3) a line for supplying a hot, liquid asphaltene stream comprising the asphaltene fraction to the prilling head; (4) a vertical height of the discharge zone sufficient to allow asphaltene discharged from the prilling head to form into liquid droplets; (5) nozzles for spraying water inwardly into the cooling zone to cool and at least partially solidify the liquid droplets to be collected in the bath and form a slurry of solidified asphaltene particles in the bath; (6) a line for supplying water to the nozzles and the bath to maintain a depth of the bath in the prilling vessel; and (7) a line for withdrawing the slurry of the asphaltene particles in the bath water from the prilling vessel. The slurrying unit can also include a liquid-solid separator such as a vibrating screen for dewatering pellets from the slurry.
In the gasification system, the atmospheric fractionator unit, the solvent deasphalting unit, the slurrying unit and the gasification unit are preferably centrally located with a plurality of the steam supply lines carrying steam to a plurality of the injection wells located away from the central location. CO2 can also be generated by and recovered from the gasification unit, and a line or lines can supply the CO2 from the gasification unit to at least one of the injection wells. A turbine can also be used for expanding a portion of the steam generated by the gasification unit to generate electricity.